The present invention relates generally to reference current circuits and more specifically to an improved programmable reference current circuit.
A reference current on an integrated circuit typically has a tolerance of .+-.20%. Many complicated circuits are provided to assure a reference current which is indepenent of voltage supply, temperature, processing variations and other physical as well as environmental characteristics. Similarly, adjustability using variable resistance and/or laser trimming have been suggested.
It is an object of the present invention to provide a unique reference current circuit which is programmable.
Another object of the present invention is to provide a reference current circuit which is readily programmable without sophisticated equipment or processing time.
These and other objects of the invention are attained by providing a current mirror which receives the actual reference current and is programmable to provide the desired reference current as a function of the actual reference current. The current mirror includes a controlled leg having a transistor and a plurality of controlling legs including a plurality of parallel connected transistors in a current mirror configuration. In addition to the primary controlled transistor, a plurality of secondary control transistors are connected with a programming element in its current path. The method of programming includes measuring the input reference current in the controlling leg and programming the individual secondary controlled legs to provide a desired output reference current which is a function of the input reference current. By controlling the current handling capacity of the plurality of controlled legs, the desired output reference current is provided. A diode is provided between the reference input current and the controlling transistor such that the input reference current can be measured without activating the current mirror. The programming of the secondary controlled legs is achieved by a programming terminal for each of the secondary legs used in combination with a reference terminal of the current mirror. Preferably, the current capacity of the controlling leg and the primary controlled leg are equal whereas the current capacity of the secondary controlled legs have a binary weighted current capacity.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.